AMCP WGW/WP13
AERONAUTICAL MOBILE COMMUNICATIONS PANEL (AMCP)
Working Group of the Whole
Montreal, Canada
21-24 May, 2002
Overview Comparison of the ADS-B Link Candidates
Presented by Brent Phillips
(Prepared by)
Michael Biggs
Federal Aviation Administration
SUMMARY
This paper provides an overview comparison, against a number of metrics, of three
candidate Automatic Dependent Surveillance – Broadcast (ADS-B) links. The paper
concludes that the potential benefits offered by the UAT indicate that it too should
undergo ICAO standardization.
1.0 Introduction
The purpose of this paper is to provide an overview comparison, against a number of metrics, of
three candidate Automatic Dependent Surveillance – Broadcast (ADS-B) links. Of the three links
of interest – 1090 MHz squitter, VHF Data Link Mode-4 (VDL-4), and the Universal Access
Transceiver (UAT) – only the former two currently have1 International Civil Aviation
Organization (ICAO) Standards and Recommended Practices (SARPs). The paper concludes that
the potential benefits offered by the UAT indicate that it too should undergo ICAO
standardization.
2.0 Metrics of Comparison
While ICAO Panel working groups tend to focus – and rightly so – on the technical aspects of
radio links, the decision to start SARPs requires examination of other factors. In particular, issues
such as spectrum availability, system cost, transition, probability of voluntary equipage, security
and other factors must also be considered.
2.1 Technical Performance
The UAT has been specifically designed to perform the ADS-B function, and as such performs
very well technically. In particular, technical comparisons of the candidate links are contained in
the report of the joint Federal Aviation Administration/Eurocontrol Technical Link Assessment
Team (TLAT), and papers presented to the Aeronautical Mobile Communications Panel (AMCP)
Working Group C (WGC). As a result of reviews of that material, WGC/3 reported:
“6.16 As a result of its review of the TLAT findings (as summarized in Attachment F), the
meeting identified the following advantages of the UAT:
1) UAT was the only candidate link to meet all TLAT-addressed performance requirements
in the future low-density scenario. Air-to-air ADS-B performance was evaluated as being
acceptable to a range of at least 150 nmi;
2) UAT met more of the performance-related link evaluation criteria addressed by the TLAT
than either of the other two candidates in the future high-density Los Angeles Basin 2020
scenario. Air-to-air ADS-B performance was evaluated as acceptable to range of 90-95
nmi;
3) For the future high-density Core Europe 2015 scenario, the number of performancerelated link evaluation criteria addressed by the TLAT that were met (or likely to be met)
by the UAT exceeded or equalled the number that were met (or likely to be met) by each
of the two systems. Air-to-air ADS-B performance was evaluated as being acceptable to a
range of 70 nmi;
4) UAT has the highest available capacity for FIS-B (after consideration of ADS-B capacity
needs) of the three candidates in the future high-density traffic scenario for the Los
Angeles Basin (FIS-B was not evaluated for the Core Europe scenario).”
“6.17 Since the TLAT published its results, UAT system changes have been developed in the
MOPS process to improve tolerance to external pulsed interference (JTIDS/MIDS and DME),
and a simulation error that penalized UAT performance in the TLAT simulation has been
corrected. (Details on these developments were provided in [WGC/3] WP10). As a result:
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Though current SARPS exist for those two links, it is expected that modifications to those SARPS will be
necessary based on ongoing technical assessment work.
2
1) Under the TLAT assumption of a clear channel (no external interference), the MOPSdefined UAT system meets all air-air and air-ground performance requirements for all
scenarios investigated by the TLAT;
2) Under a realistic worst case JTIDS/MIDS and DME, UAT still meets all requirements in
LA2020;
3) Preliminary indications for Core Europe are that performance requirements will be met
up to ranges of 100 nmi air-air.”
“6.18 This level of performance by UAT exceeds that of either of the two other links.”
Further detail, including updated performance estimates, will be presented in other papers to this
Working Group of the Whole (WGW) meeting.
2.2 Spectrum availability
At recent World Radiocommunication Conferences (WRCs), spectrum suitable for aviation
services has been under increasing pressure from other services; therefore it is highly unlikely
that new aviation bands will be allocated. As a result, new systems must be designed to utilize
existing aviation spectrum in a compatible manner with existing aviation services. Since the
ICAO spectrum handbook requires2 that surveillance systems such as ADS-B operate on channels
protected for aeronautical radionavigation services (ARNS), the problem of suitable spectrum for
that function is further exacerbated.
2.2.1 UAT: The UAT has been designed to operate in the lower portion of the 960-1215 MHz
ARNS band. This band is extensively used throughout the world for distance measuring
equipment (DME) and Tactical Air Navigation (TACAN). The band is also utilized in some
States on a non-interference basis by the military Joint Tactical Information Distribution
System/Multifunctional Information Distribution System (JTIDS/MIDS). Recognizing this heavy
use, UAT has been designed to be compatible with high-density pulsed environments. Coupled
with the selection of a UAT center frequency (978 MHz) that is unused for DME/TACAN in
much of the world due to difficulties protecting its paired VHF omni-directional range (VOR)
frequency from FM broadcast stations, it is expected that the UAT can be introduced to the band
with little/no change required to existing DME/TACAN assignments3, DME/TACAN “ramp
tester” utilization, or JTIDS/MIDS operations. In addition, from a regulatory standpoint, at least
one Administration’s responsible agency has made a preliminary ruling that UAT is an acceptable
use of the band under the existing provisions for “electronic aids to airborne navigation” as
contained in S5.328 of the International Telecommunications Union (ITU) Radio Regulations.
This should facilitate worldwide implementation of UAT as no changes to the Radio Regulations
are expected to be necessary.
2.2.2 1090 Squitter: This system is allocated to operate in the 960-1215 MHz ARNS band, on a
channel that is also heavily used for ICAO-standard secondary surveillance radar (SSR) and
aircraft collision avoidance systems (ACAS), and military identification friend or foe (IFF)
systems. This heavy utilization may limit capacity (e.g., use for ground vehicles in the airport
movement area) and/or ADS-B range of a 1090 squitter implementation, however there are no
regulatory issues with this link.
Document 9718-AN/957, Second Edition – 2000, Section 8.2.4.
DME Testing indicates that acceptable DME performance will be maintained for cochannel (978 MHz)
DME assignments even in the presence of high-density UAT environments. However future UAT
performance in high-density UAT environments will ultimately require reassigning some number of the 6
current European 978 MHz DME assignments
2
3
3
2.2.3 VDL-4: VDL-4 is intended for operation in the 108-117.975 MHz ARNS band4. That
band is extensively used throughout the world for ICAO-standard VOR, Instrument Landing
System (ILS), and the soon-to-be-implemented Ground Based Augmentation System (GBAS) for
the Global Navigation Satellite System (GNSS). Introduction of VDL-4 in that band is further
complicated by the fact that all existing/planned systems in that band are ground-based, and
therefore assignments are ground-coordinated. Airborne VDL-4 will require revisiting those
assignment procedures, and perhaps providing dedicated channels (including any necessary guard
bands) for VDL-4. This could prove very difficult to accomplish without impacting
VOR/ILS/GBAS capacity. In addition, ADS-B operations are currently not authorized in the
selected band, though efforts are underway within AMCP working group F to try to expand a
current 2003 WRC agenda item to include provisions for VHF ADS-B. In the best case,
assuming WRC and channel assignment criteria/procedures are successful, VDL-4 operations
could not begin until late 2003.
2.3 Cost
Examinations of system cost must account for a number of factors including avionics,
installation on aircraft, certification, maintenance, and cost/number/installation of ground
systems. As part of its ADS-B link decision studies, the FAA has collected cost information,
including data from potential system manufacturers. While this information is preliminary, it
does allow for comparative estimates to be made. Table 1 provides relative avionics plus
installation cost estimates (using UAT cost for that particular aircraft type as “1”), for a variety of
aircraft types.
Table 1 Preliminary Cost Comparison
Candidate
Single Link
with UAT
with 1090
with VDL-4
Air Transport Aircraft
1
N/A
1.03-1.8
N/A
UAT
1
1.03-1.8
N/A
1.3-2.4
1090
1.23
N/A
1.3-2.4
N/A
VDL-4
General Aviation Aircraft (High/Corporate)
1
N/A
1.24
N/A
UAT
1.05
1.24
N/A
1.81
1090
1.25
N/A
1.81
N/A
VDL-4
General Aviation Aircraft (Low)
1
N/A
1.46
N/A
UAT
1.21
1.46
N/A
Note 5
1090
1.58
N/A
Note 5
N/A
VDL-4
Notes: 1. Cost is normalized to UAT single installation system cost for each aircraft category
2. Costs for air transport reflect average including forward-fit and retro-fit
3. For the analysis, all dual-link scenarios considered included 1090 squitter as one of
the links
4
There has also been consideration of utilizing portions of the 118-137 MHz aeronautical mobile (R)
service (AM(R)S) band for VDL-4. Aside from the “surveillance should be in ARNS bands” issue
discussed above, the 118-137 MHz AM(R)S band is also highly congested in much of the world. In fact,
that congestion is the driving force behind the 8.33 kHz channel splitting ongoing in Europe, and the
continued development of the ICAO standard VDL-3 in the United States.
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4. All cost data reflects vendor estimates only for avionics + installation
5. 1090/Mode-4 configuration not considered for general aviation due to cost
constraints
2.3.1 UAT: As shown in Table 1, UAT is approximately the same cost as 1090 squitter for air
carrier installations, and considerably cheaper (approximately 21% less) for general aviation.
UAT is 23% to 58% cheaper than VDL-4 depending on platform. UAT ground systems are
expected to provide similar cost benefits as they are functionally the same as the UAT airborne
system and should be able to utilize existing inventory of inexpensive omni-directional DME
antennas.
2.3.2 1090 squitter: 1090 squitter is approximately the same cost as UAT for air carrier
installations, and about 21% more expensive for general aviation. 1090 squitter is at least 17%
cheaper than VDL-4 for any installation considered.
2.3.3 VDL-4: VDL-4 is more expensive than either UAT or 1090 squitter for any of the
installations considered
2.3.4 Certification: Certification costs were handled separately in the analysis, but are estimated
to be comparable for UAT and 1090 squitter, and approximately 33% more for VDL-4 or any of
the dual-link configurations.
2.4 Transition
Clearly the introduction of, and benefits due to, ADS-B will not occur “overnight”. During this
introduction period, it is highly desirable that the introduction of the ADS-B system not cause
impact on other, existing, aviation systems. This greatly improves cost-benefit analyses as the
“benefits” of ADS-B are allowed to accrue without “cost” to current operations.
2.4.1 UAT: As noted above, UAT intends to utilize a DME channel that is not used in much of
the world, and is in-fact noted in Annex 10 as being assignable only “for emergency use” because
its paired VHF channel is “not scheduled for assignment”. With its demonstrated compatibility
with expected environmental sources, no changes should be required in order to implement the
UAT system beyond the eventual retuning of up to 6 “on channel” European DME stations. In
fact, initial UAT trials can be accomplished with no change to DME usage by capturing some of
the unused ADS-B system self-interference capacity – needed only in high-density future ADS-B
scenarios – to accommodate DME interference. As a result, no transition issues are foreseen.
2.4.2 1090 squitter: As this technology operates on an existing radio link, it is expected that the
only transition issues would be the certification necessary to ensure squitter use does not impact
the operation of the safety-of-life SSR and ACAS.
2.4.3 VDL-4: Transition issues for this system are difficult to determine as it is unclear how
many channels will be required, and no channel planning criteria have been defined to ensure the
system is compatible with in-band VOR/ILS/GBAS, and adjacent-band VHF communications.
Given the ground-coordinated nature and heavy usage of the intended band of operation, and the
expected need to clear some number of existing channels, considerable channel reassignments
may be necessary. Further study is required to determine the magnitude and feasibility of such
changes. In addition, as noted above, changes to the ITU Radio Regulations will be required to
enable VDL-4 use in the ARNS band. These regulatory changes cannot be accomplished prior to
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late 2003, and may be hampered by the lack of studies on the compatibility of VDL-4 with
adjacent-band broadcast services5.
2.5 Probability of voluntary equipage
It is easily recognized that the maximum benefits of ADS-B cannot be realized until all aircraft
are equipped. As a result, to avoid mandates, especially in States with large populations of
general aviation aircraft, the probability that users will voluntarily equip with the ADS-B link
becomes a big factor in achieving those benefits. In general, that probability is directly
influenced by (a) cost of the system, and (b) services offered by the system. As the former is
addressed in section 2.3 above, this section will concentrate on a comparison of services – beyond
simply ADS-B – offered by candidate links.
2.5.1 UAT: In recognition of the need to build user acceptance, UAT has been expressly
designed to provide additional services beyond simply ADS-B. In particular, the system offers 32
message slots per second that can be “assigned” to ground facilities. The message protocols for
these slots are flexible enough to allow a variety of message types, however current plans call for
using that capability to provide Flight Information Services (FIS) such as weather data. In
addition, the slotted nature of those ground transmissions, together with the surveyed position of
the ground systems, allow for the potential of using the message arrival relationship as a form of
position determination. Though this capability is not addressed directly in the current receiver
minimum operational performance standards (MOPS) work, care has been taken to ensure the
UAT MOPS will support position determination in an external application. It is important to note
that these additional features can be realized with no impact on UAT ADS-B performance.
2.5.2 1090 Squitter: 1090 squitter is seen as an attractive ADS-B alternative for the air carrier
community because it offers a yet-to-be-determined evolution of the Mode S and TCAS systems
already in use in most air carrier aircraft. While 1090 squitter does not offer as much capacity as
UAT for providing data services such as FIS-B, it may be able to support a basic set of such
services. It would however have limitations in the highest traffic density environments.
2.5.3 VDL-4: While VDL-4 is proposed to support additional services like FIS-B and even two
way data link, additional channels are required to support these services. The four channel
avionics configuration evaluated for the Eurocontrol/FAA technical Link Assessment Team
(TLAT) study made no allowance for two way data link traffic and allocated significantly less
capacity for FIS-B than UAT.
2.6 Security
The ability to support some form of ADS-B message validation will be an important feature in
preventing “spoofing”6 of an ADS-B system.
2.6.1 UAT: The UAT system architecture allows a receiver to accurately measure the time of
propagation of ADS-B messages. This allows a range to target comparison to be made between
the range indicated by the propagation time and the range indicated by broadcast ADS-B position.
Flight tests of UAT have shown that the UAT waveform and demodulation process can support
timing measurements with an accuracy of <0.2 nmi. . Recognizing the importance of such a
5
This includes both the ability of VDL-4 receivers to tolerate adjacent-band broadcast signals, and
ensuring that airborne VDL-4 transmissions do not interfere with broadcast service receivers.
6
For example interrupting airport operations by broadcasting the presence of a false/phantom target in the
vicinity of an active runway.
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capability, the MOPS work has also taken care to ensure UAT standards will support independent
range validation in an external application.
2.6.2 1090 Squitter: The 1090 squitter system relies on ACAS to perform a range validation,
however this will be limited in range to 15-30 nmi, and only for platforms which also have
ACAS.
2.6.3 VDL-4: VDL-4 could support an independent range validation similar in principle to that
of UAT. However the narrower bandwidth of the signal will result in less accurate
measurements. In addition, we are aware of no flight data available regarding this feature, nor of
plans to implement it in the VDL-4 architecture.
2.7 Other Factors
2.7.1 Future Systems: A strategic benefit of developing ICAO standards for UAT is that it
establishes a physical layer waveform that could be reused for future aeronautical systems. The
band between 960-976 in particular, currently reserved for national allotments, could be
recovered and coordinated for future “broadband” aeronautical applications. Use of this band
would offer 1 Mbps channels not practical at VHF yet offer propagation characteristics superior
to that in the 5000-5250 MHz band.
2.7.2 Operational experience: ICAO standardization of UAT now would benefit from the
significant operational experience gained with the UAT design in the FAA’s Capstone program.
Over 150 aircraft in Western Alaska are using an early version of UAT for operational air-ground
ATC service. These airborne installations are also supporting air-air situational awareness and
ground-air uplink of weather information (FIS-B). UAT is the only data link we are aware of to
experience the rigors of operational use. The MOPS development is making some changes to the
system based on the Capstone implementation, however the fundamental principles of simplicity
and robustness of the channel structure and media access are preserved, and actually enhanced.
3.0 UAT as compared to the Combination of 1090 ES and VDL-4
Preliminary proposals have been made to form a combination of 1090 squitter and VDL-4 as
complementary links for purposes of forming a “hybrid” system for ADS-B, TIS-B and FIS-B
that collectively has better performance than each system separately. With such a proposal the
question arises, how does performance of this hybrid system compare with that of UAT alone? A
detailed technical assessment of the hybrid combination cannot be made because a detailed
configuration for this coupling of the two systems has not been proposed. However, assuming it
is possible to define a 1090 squitter and VDL-4 hybrid configuration with equivalent technical
performance to UAT, the following non-technical aspects must be considered in light of the
analysis in this paper:



The simplicity, logistical, and aircraft integration advantage of a single link ADS-B,
TIS-B, and FIS-B system
The cost advantage of a single link system both for avionics and ground stations vs
the cost for the 1090ES/VDL4 hybrid (see Table 1, where they hybrid system was
estimated to cost 180-240% as much as the UAT alone, and was not even considered
for general aviation due to cost concerns).
Should near-term ADS-B application rollout considerations argue for a multi-link
approach (e.g., while UAT SARPs are being developed), a 1090 squitter/UAT
combination appears to be more cost effective than a 1090 squitter/VDL4 hybrid.
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4.0 Conclusion
As outlined above, UAT offers significant advantages in the provision of ADS-B when compared
to the other candidate links. These advantages are especially evident when the general aviation
community is considered; a community that is expected to gain considerable benefits from ADSB as they are usually not equipped with other situational awareness systems such as ACAS.
Review of this information, together with the superior technical performance of the UAT,
resulted in the WGC recommendation to begin UAT SARPS development.
5.0 Recommendation
The WGW is invited to note the benefits of UAT and recommendation of WGC, and to
recommend to the Air Navigation Commission the initiation of ICAO SARPs development for
the UAT.
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